1. Field of the Invention
The field of the invention is methods and apparatus for detecting and reproducing sound.
2. Description of the Background Art
Extensive physical and behavioral studies have revealed that the external ear (including torso, head, pinna, and canal) plays an important role in spatial hearing. It is known that the external ear modifies the spectrum of incoming sound according to incident angle of that sound. It is further known that in the context of binaural hearing, the spectral difference created by the external ears introduces important cues for localizing sounds in addition to interaural time and intensity differences. When the sound source is within the sagittal plane, or in the case of monaural hearing, the spectral cues provided by the external ear are utilized almost exclusively by the auditory system to identify the location of the sound source. The external ears also externalize the sound image. Sounds presented binaurally with the original time and intensity differences but without the spectral cues introduced by the external ear are typically perceived as originating inside the listener's head.
Functional models of the external ear transformation characteristics are of great interest for simulating realistic auditory images over headphones. The problem of reproducing sound as it would be heard in three-dimensional space occurs in hearing research, high fidelity music reproduction, and voice communication.
Kistler and Wightman describe a methodology based on free-field-to-eardrum transfer functions (FETF's), also known as head related transfer functions (HRTFs), in a paper published in the Journal of the Acoustical Society of America (March, 1992) pp. 1637-1647. This methodology analyzes the amplitude spectrum and the results represent up to 90% of the energy in the measured FETF amplitude. This methodology does not provide for interpolation of the FETF's between measured points in the spherical auditory space around the listener's head, or represent the FETF phase.
For further background art in the relevant area of auditory research, reference is made to the Introduction portion of our article, "External Ear Transfer Function Modeling: A Beamforming Approach", published in the Journal of the Acoustical Society of America, vol. 92, no. 4, Pt. 1 (Oct. 30, 1992) pages 1933-1944.